Lens elements are used, for example, in order to realize autostereoscopic display devices. Such devices comprise a liquid crystal display panel for example of the active matrix type that acts as a spatial light modulator to produce the display image. The display panel has an orthogonal array of display pixels arranged in rows and columns. In practice, the display panel comprises about one thousand rows and several thousand columns of display pixels.
The structure of the liquid crystal display panel is entirely conventional. In particular, the panel comprises a pair of spaced transparent glass substrates, between which for example an aligned twisted nematic or another liquid crystal medium is provided. The substrates carry patterns of transparent indium tin oxide (ITO) electrodes on their facing surfaces. Polarizing layers are also provided on the outer surfaces of the substrates.
Each display pixel comprises opposing electrodes on the substrates, with the intervening liquid crystal medium there between. The shape and layout of the display pixels are determined by the shape and layout of the electrodes. The display pixels are regularly spaced from one another by gaps. Each display pixel is associated with a switching element, such as a thin film transistor (TFT) or thin film diode (TFD). The display pixels are operated to produce the display image by providing addressing signals to the switching elements, and those skilled in the art will know suitable addressing schemes.
The display panel is illuminated by a light source comprising, in this case, a planar backlight extending over the area of the display pixel array. Light from the light source is directed through the display panel, with the individual display pixels being driven to modulate the light and produce the display image.
The display device of prior art also comprises a lenticular sheet, arranged over the front side of the display panel, which performs a view forming function. The lenticular sheet comprises a row of lens elements extending parallel to one another. The lens elements are in a form of planoconvex lenses, and they act as a light output directing means to provide different images, or views, from the display panel to the eyes of a user positioned in front of the display device.
The autostereoscopic display devices of prior art are capable of providing several different perspective views in different directions. In particular, each lenticular element overlies a small group of display pixels in each row. Accordingly, the lens element projects each display pixel of a group in a different direction, to form the several different views. As the user's head moves from left to right, his/her eyes will receive different ones of the several views, in turn.
It has also been proposed to provide electrically switchable lens elements, which enable the display to be operated in a 2D or 3D mode.
In this connection, the array of electrically switchable lenticular elements, which can be employed in such device, comprises a pair of transparent glass substrates with transparent electrodes formed of indium tin oxide (ITO) provided on their facing surfaces. An inverse lens structure formed using a replication technique, is provided between the substrates adjacent to an upper one of the substrates. A liquid crystal medium is provided between the substrates adjacent to the lower one of the substrates. Surfaces of the inverse lens structure and the lower substrate that are in contact with the liquid crystal material are provided with an orientation layer for orientating the liquid crystal material.
When no electric potential is applied to the electrodes, the refractive index of the liquid crystal material is substantially higher than that of the inverse lens array, and the lenticular shapes therefore provide a light output directing function.
When an alternating electric potential of approximately 50 to 100 volts is applied to the electrodes, the refractive index of the liquid crystal material is substantially the same as that of the inverse lens array, so that the light output directing function of the lenticular shapes is cancelled. Thus, in this state, the array effectively acts in a “pass through” mode.
Further details of such a structure and the operation of arrays of switchable lenticular elements suitable for the use in such display devices can be found in U.S. Pat. No. 6,069,650 A1.
An example of an autostereoscopic display device, which has both 2D and 3D modes of operation, is known from WO 2010/136951 A1, which disclosure is incorporated by reference to this application.
This device comprises a display panel having an array of display pixels for producing a display image and the display pixels being arranged in rows and columns. A lens arrangement covers thereby the display panel and comprises switchable lens elements, which are formed of a blue phase LC medium, and a replica structure, which are formed of a non-switchable isotropic material. The disclosed lens arrangement directs the output from different pixels to different spatial positions and enables an autostereoscopic image to be viewed. The device further comprises a controller adapted to switch the blue phase material to an isotropic state for the 2D mode and to a birefringent state for the 3D mode, and wherein in the 2D mode, the refractive index of the blue phase material matches the refractive index of the non-switchable isotropic material.
As described above, the isotropic state is used for the 2D mode, which highly improves the 2D mode in comparison to the electric-field aligned mode described before because the residual diffraction in the 2D mode is reduced to a minimum, by using the optically isotropic phase of the blue phase medium.
The existence of the so-called blue phase of a liquid crystal medium has been recognized for many decades. However, this phase was associated with a very narrow temperature range. The blue phase is caused by defects that occur at regular distances in three spatial dimensions form a cubic lattice. A regular three-dimensional lattice of defects within a chiral liquid crystal thus forms blue phases. Since the spacing between the defects of a blue phase are in the range of the wavelength of light (several hundred nanometers), for certain wavelength ranges of the light reflected from the lattice constructive interference occurs (Bragg reflection) and the blue phase reflects colored light (note that only some of the blue phases actually reflect blue light). The blue phase arises when the chiral LC material is warmed from the cholesteric phase or cooled from the isotropic phase.
In 2005, researchers from the Centre of Molecular Materials for Photonics and Electronics at the University of Cambridge reported their discovery of a class of blue-phase liquid crystals that remain stable over a range of temperatures as wide as 16-60 Celsius, as published in Nature, 436, pages 997-1000. It has been shown that these ultrastable blue phases could be used to switch the color of the reflected light by applying an electric field to the material, and that this could eventually be used to produce three-color (red, green, and blue) pixels for full-color displays. The new blue phases are made from molecules in which two stiff, rod-like segments are linked by a flexible chain.
In addition, a prototype of a blue phase LCD panel has been produced and publicized. The blue phase mode does not require LC alignment layers, unlike conventional TFT LCD technologies such as Twisted Nematic (TN), In-Plane Switching (IPS) or Vertical Alignment (VA). The blue phase mode can make its own alignment, eliminating the need for any mechanical alignment and rubbing processes. This reduces the number of required manufacturing steps, resulting in savings on production costs.
In a blue phase based LC-display for TV applications it is not the selective reflection of light according to the lattice pitch (as a result of Bragg reflection) that is used for display of visual information, but an external electric field induces a birefringence in the LC via the Kerr effect. That field-induced birefringence becomes apparent as a change of transmission when the blue phase Mode LC layer is placed between crossed polarizers.
By applying a blue phase LC medium to a switchable lens element, the need for alignment layers is avoided, as well as the need for LC rubbing, because the isotropic mode is angle and polarization independent. The structure and manufacturing process of such a lens element is therefore simplified.
In this regard, some compounds and media have been reported which possess a blue phase between the cholesteric phase and the isotropic phase that can usually be observed by optical microscopy. These compounds or compositions, for which the blue phases are observed, are typically single mesogenic compounds or mixtures showing a high chirality. Generally, the blue phases observed only extend over a very small temperature range, which is typically less than 1 degree centigrade wide, and/or the blue phase is located at rather inconvenient temperatures.
The autostereoscopic display device according to WO 2010/136951 A1 suggests e.g. the disclosed media in WO 2005/075603 A1 or WO 2007/147516 A1 as a suitable media.
Modern lens element applications require fast switching times, which can be achieved by higher values for the dielectric anisotropy and favourably lower viscosities of the blue phase LC media. In addition to that, the media have to exhibit a high UV stability, since they are directly exposed to sunlight. Moreover, the needed light modulation media have to exhibit a blue phase, which is as wide as possible and which is conveniently located for the mentioned applications together with a good stability at deep temperatures for outdoor applications, i.e. in cameras.
Thus, there is a need for further liquid crystal media, which can be operated in a switchable lens element, which are operated at a broad range of temperatures where the media is in the blue phase, which provide besides high absolute values for Δn, good thermal and UV stability and a blue phase over a wide temperature range, and which also provide the following technical improvements:                a reduced operating voltage,        a reduced temperature dependency of the operating voltage and        high values for the dielectric anisotropy together with favourably low viscosities.        